Seeing the Sky with Different Eyes

By Sean Carroll | August 28, 2008 4:42 pm

I just got back from the Cosmo-08 conference in Madison, which was great fun (and I’m sorry I had to miss the last couple of days). But just because I’m traveling, doesn’t mean that science stops happening. It just means I might be late in blogging about it, if I were moved to do so, which in this case I am.

The big news is that the Gamma-Ray Large Area Space Telescope, a satellite observatory launched back in June, reached two milestones: (1) it got a name change, from GLAST to the Fermi Gamma-Ray Space Telescope (on the theory that not enough things are named after Fermi), and (2) they released the first new picture of the gamma-ray sky! And here it is; click for higher resolution.

You can clearly see the Galactic plane, of course, as well as a few objects that shine brightly in gamma-rays — a handful of pulsars, and one distant blazar. GLAST Fermi will be cranking out the science over the next several years, from down-and-dirty astrophysics to searches for annihilating dark matter. See Andrew Jaffe, Phil Plait, and symmetry breaking.

Meanwhile, some of the folks who brought you the Bullet Cluster have now come up with MACSJ0025.4-1222 (or MAC-Daddy-J, as pundits have suggested).

It’s a similar story — two giant clusters of galaxies smacked into each other, allowing their dark matter to separate from the hot ordinary matter in between. Gravitational lensing lets you figure out where the dark matter is, while X-ray observations reveal the ordinary matter. The Bullet Cluster was pretty darn convincing, but it’s a scientific truism that nothing ever happens just once, so it’s nice to see that magic repeated.

Finally, I wanted to mention something that is somewhat old news, but that somehow had escaped my attention until Dan Hooper‘s nice talk at Cosmo-08. That is the WMAP Haze, a phenomenon originally noticed by Douglas Finkbeiner. The WMAP satellite, trying to observe primordial temperature perturbations in the cosmic microwave background, measures several different frequencies, to help correct for foregrounds. The CMB isn’t the only thing that emits microwaves, but nearby dusty astrophysical sources generally depend on frequency in different ways, so one can try to remove their effects by seeing how the maps at different frequencies compare with each other. Some people, apparently, are actually interested in those dusty foregrounds, so they try not only to remove them, but to understand them. And Finkbeiner claims that when we remove all of the foregrounds that we know how to explain, and mask out the parts of the galaxy that are simply too bright to deal with, we are left with this:

There is some mysterious emission near the center of the galaxy, dubbed the “WMAP haze.” There is an explanation on the market, which is what Hooper’s talk was about — the haze could come indirectly from dark matter! Dark matter particles annihilate, so this story goes, giving off a bunch of lighter particles, including electrons and positrons. These electrons and positrons swirl around in the galactic magnetic field, giving off synchrotron radiation, which is what we see as the haze.

True? Plausible? Crazy? I don’t know. The good news is that the dark matter model required to make it work is not thrown together just to fit this result — it’s a fairly vanilla model of weakly-interacting massive particles. The bad news is that it’s hard to understand these dusty foregrounds, and difficult to be sure that you’ve accounted for all of the mundane ones.

The great news is that this is exactly the kind of thing that GLAST Fermi will test, by looking for the high-energy gamma rays that should also be emitted by annihilating dark matter. So stay tuned for some possibly exciting dark matter news, right around the corner.

  • Matt

    Thanks Sean!

    I love your astrophysics summaries. No one does succinct and cogent like you.

  • Kurt

    So is this another nail in the coffin for the MOND explanation?
    I have read one or two arxiv papers that try to explain bullet cluster with MOND but they are reaching it seems.

  • cq

    How odd that the bright parts of the galaxy look like Lake Superior!

  • Lab Lemming

    Which way is up?
    I mean, it is obvious that the orientation is so that the plane of the galaxy goes across, but which direction is galactic North? My star chart doesn’t do galactic co-ordinates…

  • Eric Habegger

    I think this new data is very interesting but it is better not to superimpose one’s own thought processes on that data. You said “Dark Matter anhillilates – so this story goes.”

    That is a very big jump and depends on the usual quantum mechanical machinations that are not really rigorous in bright sunlight. It is only one interpretation that many people, including myself, don’t agree with. Let people enjoy the data unfiltered.

  • Lawrence B. Crowell

    As much dark matter exists in galactic halos we might question why dark matter would annihilate near the galactic centers and not in the halo. I suspect that this galactic central glow is not related to dark matter.

    Lawrence B. Crowell

  • Gabe


    In fact it is expected that the DM density near the center of the galaxy is much higher than in the rest of the halo. In the Hooper, Finkbeiner and Dobler paper that Sean linked to, one of the first things they do is constrain the halo profile that the Haze would require. The “cusped” profile necessary falls roughly between the Navarro-Frenk-White and the Moore et al. N-body simulation profiles. So the Haze doesn’t require a shocking distribution of DM.
    The higher density (and thus higher annihilation rate) is also why there are other experiments trying to detect gamma rays and other particles produced by DM annihilations in the galactic center.

  • Lawrence B. Crowell

    Most of the gamma rays in the universe, and the galactic center, are from e-e^+ annihilations and related processes. If dark matter is associated with neutrolinos, the gamma rays might be far upscale in energy from there in the multi TeV range. Another candidate for dark matter is strangelet matter, but that might be completely stable.

    It is not clear whether this is something we should hold our breath over.

    Lawrence B. Crowell

  • Rich

    Galactic foregrounds are becoming more and more important to understanding how to interpret various astronomical data. It’s really striking how areas of CMB astrophysics, such as polarisation and the search for primordial non-Gaussianity, are becoming limited by our understanding of the effects of the foregrounds.

    Fermi might well be a strong driver for trying to develop better models of the “known” physics that gives us gamma ray emissions in the galactic place, so we can distinguish between that and new effects (eg. dark matter decays).

  • Shantanu

    Sean, does the bullet cluster rule out Luc Blanchet’s explanation of MOND
    as advocated in this and references

  • Sean

    Shatanu, I haven’t read the paper, so I can’t answer in detail. Bullet-Cluster-type measurements imply that there is some “dark” form of energy that acts essentially like dark matter. So the explanation can’t be only modified gravity. But some particular theory of “modified gravity” might include extra forms of energy-momentum that mimic dark matter, and therefore be consistent with the data.

  • Fermi-Walker Public Transport

    Synchrotron emission is also polarized, sometimes in the tens of percent, so that would be another way of verifying if that is the emission mechanism. However
    polarization can also be masked by beam size effects (resolution) and by an ionized medium along the line of sight. Still, it may be worth checking and if found, would be good support.

  • Shantanu

    Sean, I too haven’t read all of Blanchet’s papers on this subject and my information may be outdated.
    But his first paper posited that of dark matter consists of a gravitational dipole,
    then you can easily explain Milgrom’s law on galactic scales (without
    invoking MOND). The downside is that dark matter now consists of particle
    with negative mass. Does the Bullet cluster rule out these models?

  • Pingback: Something’s in the air | Cosmic Variance | Discover Magazine()


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About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] .


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